1. Field of the Invention
The present invention relates to an optical detection sensor that detects a physical quantity such as pressure, temperature, or the like by measuring a change in light intensity. Specifically, the present invention relates to an optical detection sensor that allows for a highly accurate detection of a displacement amount.
2. Description of the Related Art
There has since been known an electric sensor as a sensor that measures a physical quantity such as an amount of displacement, temperature, pressure or the like of an object subjected to measurement. However, since a measurement signal (electric signal) is transmitted to a remote area through an electric wire, which is prone to influence of electromagnetic noises, there has existed a problem of measurement errors. In contrast, an optical sensor is capable of transmitting a signal without being influenced by electric noises, since a measurement signal is transformed into an optical signal, which is then transmitted through an optical fiber, thereby providing advantages of reduced measurement errors and extremely accurate measurement.
There is such an optical physical quantity sensor that employs a distortion-induced body (for example, a bourdon tube or the like) having a mechanism of being distorted by pressure, then transforms the pressure into distortion, and detects the distortion through a fiber grating (FBG). In addition, there is an optical sensor in which a reflection plate (such as a diaphragm) that displaces itself by pressure is securely attached on an end face of an optical fiber and there is detected a positional change of a reflection light that has once been emitted from the end surface of the optical fiber to be reflected from the reflection plate, thereby to measure the pressure through the position.
As a method of detecting the positional displacement, there are some methods employing a detection of interference or beat frequency caused by plural rays of reflection light (for example, Japanese Patent No. 3304696 (page 15, FIG. 2), or a detection of a resonance frequency occurring in an oscillator secured on a reflection plate (for example, Japanese Patent Application Laid-open Publication No. 2003-2146966 (page 4, FIGS. 1-4). However, these methods have difficulties in signal processing and need complicated arithmetic processing.
In contrast, there is a method of measuring intensity change of reflection light as a simple practical method. This method enables to configure a measurement apparatus using inexpensive materials and components and also to process an obtained signal extremely simply. On the other hand, this method is accompanied by a disadvantage arising from some factors except for a positional displacement of the reflection plate, which is unavoidable for a detection of light intensity. One example of the disadvantage is that a change in light intensity, for example, due to transmission loss is overlapped on a measurement value. However, such a disadvantage can be overcome to improve measurement accuracy by reducing a transmission loss or obtaining an intensity ratio of plural rays of reflection light for compensation.
For example, Japanese Examined Patent Application Publication No. H06-8724 (pages 3 to 4, and FIGS. 1 and 2) discloses a configuration in which there are parallelly secured a first and a second optical fiber, which each have at least one different parameter such as a core diameter, a core refraction index distribution, an aperture number (NA) or the like, so that an end face thereof faces a reflection surface of a reflection plate and the longitudinal direction of the optical fibers are parallel with a normal line to the reflection surface. According to this configuration, light emitted from the end face of the first optical fiber is reflected and then received by the second optical fiber; light emitted from the second optical fiber is reflected by the reflection plate and then received by the first optical fiber; the two beams of reflection light respectively detected through the two optical fibers are each split by a splitter; an intensity ratio is calculated after photoelectric conversion of the respective light is conducted; and thus the ratio is used so as to compensate an influence incurred by a transmission loss.
In addition to the above measurement methods, there are a method in which light beams emitted from respective optical fibers are reflected by a reflection plate so as to return thereto and measured; another method in which two optical fibers as a light emitter and two optical fibers as a light detector, all of which are of the same type, are used and an intensity ratio of each light beam is obtained by not using a light splitter (See, U.S. Pat. Nos. 6,433,350 and 4,479,717, France Patent No. 2399000, and Japanese Patent Laid-open Publication No. II10-9813, or the like).
In addition, U.S. Pat. No. 4,996,418 (FIGS. 1, 15, and 16) discloses a technique in which a plural of multi-mode fibers that can easily be connected optically with a light emitting diode and enjoy a strong light intensity through their own large core diameter are used and secured so as to be at an angle of Φ (0°≦Φ≦5°) with respect to a normal line to a surface of liquid subjected to the measurement, thereby detecting a displacement of the liquid surface.
However, the technique disclosed in Japanese Examined Patent Application Publication No. H06-8724 has difficulties in selecting measurement sensitivity. This is because special optical fibers having different parameters need to be manufactured when intensity change in the reflection light (measurement sensitivity) occurs in accordance with a change in distance between the optical fibers and the reflection plate, since the longitudinal direction of the optical fibers is parallel with the normal line to the reflection surface. In addition, since the end face of the optical fibers lies at a right angle with respect to the longitudinal direction, there is caused a Fresnel reflection between the end face of the optical fiber and an air layer, and thus measurement accuracy deteriorates through a variation in detection light intensity, an increase in intensity loss, or the like. Moreover, there is easily caused a multiple reflection between the end face of the optical fiber and the reflection plate, thereby deteriorating measurement accuracy.
By the way, since it has since been important to obtain a large light intensity in a system employing light intensity change, use of a multimode optical fiber is effective as disclosed in U.S. Pat. Nos. 4,996,418 and 5,068,527. However, multimode fibers have a problem in that they are influenced by bending, which causes propagation-mode shifts thereby to easily change an NA and loss Use of a light-emitting diode (LED) as a light source is advantageous in terms of costs and polarization dependence. However, since an LED has a temperature dependence in which a shape of emission spectrum changes in accordance with a temperature change, specifically light intensity reduces with an increase in LED temperature and a peak wavelength shifts toward longer wavelengths, use of an optical coupler and an optical filter having a wavelength dependence lead to fluctuation of measurement values, thereby increasing measurement errors. Additionally, use of an optical fiber having a large dependence of NA on wavelength causes a large change in NA due to wavelength changes in LED, leading to a change in a beam shape of the exiting light and an amount of reflection light received, thereby changing a connection efficiency with the reflection surface. Since such a fluctuation in connection efficiency cannot be compensated, measurement accuracy is inevitably deteriorated Although a bending loss does not influence significantly when the optical fiber is used in a digital communication such as an optical communication, it has a large influence leading to a deterioration of measurement accuracy when the optical fiber is used in analog transmission.
The present invention has been made in view of the above disadvantages and the objective thereof is to provide an optical detection sensor that is free from an influence incurred by a transmission line bending, can suppress a fluctuation in measurement caused by an NA change of an optical fiber through a change in wavelength spectrum of a light source, and realizes an accurate measurement.